Strange white faces only visible in weight paint? Invisible when ported into UE4
I've figured it out?!

For whatever reason choosing to open image, and bringing in the same texture I was using and reassigning my uv's to that fixed the problem. 

The white faces have dissapeared.

Note:
I did delete faces previously and make new ones, which were where the polygons had turned white. I don't understand why this whole thing happened because of that though..